Linearity and dynamic range for complementary metal oxide semiconductor active pixel image sensors

ABSTRACT

A method and structure for a complementary metal oxide semiconductor active pixel sensor device having a photodetector, a sensing node electrically connected to the photodetector, an output connected to the photodetector, and a voltage-independent capacitance device connected between the sensing node and the output. The voltage-independent capacitance device provides a capacitance independently of a voltage on the sensing node. The voltage-independent capacitance device can be a voltage-independent capacitor, an electrode-electrode capacitor, or a common source amplifier and should have a capacitance larger than the capacitance of the sensing node. The voltage-independent capacitance device lowers an overall voltage-dependent capacitance of the APS.

FIELD OF THE INVENTION

The present invention generally relates to complementary metal oxidesemiconductor (CMOS) active pixel sensors (APS) and more particularly toan improved pixel sensor that has increased linearity as a result ofadditional voltage-independent capacitance.

BACKGROUND OF THE INVENTION

CMOS APS are solid state imagers where each pixel contains aphoto-sensing means, reset means, charge conversion means, select means,and all or part of an amplifier. APS devices have the advantages ofsingle supply operation, lower power consumption, x-y addressability,image windowing, and the ability to effectively integrate signalprocessing electronics on-chip, when compared to CCD sensors.

In order to build high resolution, small pixel APS devices for digitalcameras it is necessary to use sub-μm CMOS processes in order tominimize the area of the pixel allocated the active components in eachpixel. In order to achieve good signal to noise performance it isimportant to hold as many photoelectrons as possible within the pixel.In typical APS pixel architectures the integrated photoelectrons areconverted to a voltage in each pixel. This charge to voltage conversionregion is typically a diode, either the photodiode or an isolatedfloating diffusion. It is the parasitic capacitance of the charge tovoltage conversion region that determines the maximum number ofelectrons that can be contained within the region. Sub-μm CMOS processesare typically operated at low supply voltages, 3.3V and below, hence thereset level and the voltage swing that can be accommodated in the chargeto voltage conversion region is limited by the supply voltage. Since thesupply voltage is low, the signal swing on the charge to voltageconversion region is a large compared to the reset level. Since thecapacitance of the diode that forms the charge to voltage conversionregion is a function of the voltage across the diode, and the signalswing is large compared to the total voltage across the diode at reset,the capacitance of the diode changes substantially from the reset level,(or dark signal), to the saturation level, (or bright signal). Intypical APS pixel architectures the capacitance at reset is smaller thanthe capacitance at saturation. This produces a non-linear transferfunction. It is very important to have a linear transfer function forcolor image sensors. Non-linearity in the sensor response can degradethe color fidelity of the image. Response linearity has been optimizedfor CCD image sensors. APS are much less linear that CCD's.

In addition to poor linearity, APS sensors can also suffer from lowcharge capacity as a result of the reduced supply voltages in sub-μmCMOS processes. For the same pixel size, CMOS APS sensors have lowercharge capacity compared to CCD image sensors due to the larger supplyand clock voltages used on CCD image sensors.

One approach to providing an image sensor with the linearity of a CCDand the advantages of an APS device is to reduce the effect of thevoltage dependent capacitance of the charge to voltage conversion regionof an APS device. This invention does so by providing a voltageindependent capacitor in parallel with the diode capacitance of thecharge to voltage conversion region. This can also be used to improvethe charge capacity of an APS device.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide astructure and method for a complementary metal oxide semiconductoractive pixel sensor device having a photodetector, a charge to voltageconversion node, an amplifier input connected to the charge to voltageconversion node, and a voltage-independent capacitance connected inparallel with the charge to voltage conversion node. Thevoltage-independent capacitance provides a capacitance that is not afunction of charge placed on the charge to voltage conversion node. Thevoltage-independent capacitance can be an electrode-electrode capacitor,or the input capacitance of an amplifier.

The invention also comprises a method of manufacturing a complementarymetal oxide semiconductor active pixel sensor device which includes aphotodetector, a charge to voltage conversion node, an amplifier inputconnected to the charge to voltage conversion node, and avoltage-independent capacitance connected in parallel with the charge tovoltage conversion node. The voltage-independent capacitance provides acapacitance that is not a function of charge placed on the charge tovoltage conversion node. The voltage-independent capacitance can be anelectrode-electrode capacitor, or the input capacitance of an amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a schematic diagram of an active pixel sensor pixel;

FIG. 2 is a schematic diagram of a second active pixel sensor pixel;

FIG. 3 is a graph illustrating the linearity of voltage output by activepixel sensor pixels shown in FIGS. 1 and 2;

FIG. 4a is a schematic diagram of an active pixel sensor utilizing avoltage-independent capacitor,

FIG. 4b is a schematic diagram of an active pixel sensor utilizing avoltage-independent capacitor;

FIG. 5a is a schematic diagram of an active pixel sensor pixel utilizinga common source amplifier; and

FIG. 5b is a schematic diagram of an active pixel sensor pixel utilizinga common source amplifier.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic diagram that illustrates a CMOS APS pixel 5. Asshown in FIG. 1, the cell 5 includes a photodiode 10, a transfertransistor 11 with a transfer gate TG, whose source is connected to thephotodiode, and a reset transistor 13 with a reset gate RG, whose drainis connected to the voltage supply VDD 14. The drains of the transfertransistor 11 and the source of the reset transistor 13 form a floatingdiffusion region FD 12 which functions as a charge to voltage conversionnode. The floating diffusion region 12 is connected to the gate of theinput transistor 15 of a source follower amplifier. The source of theinput transistor 15 is connected to the drain of the row selecttransistor 16, and the source of row select transistor 16 is connectedto the column bus 17.

Operation of active pixel sensor cell 5 is performed in three steps: areset step, where cell 10 is reset from the previous integration cycle;an image integration step, where the light energy is collected andconverted into an electrical signal; and a signal readout step, wherethe signal is read out.

Referring to FIG. 1, during the reset step, the gate of reset transistor13, and transfer transistor 11 is briefly pulsed with a reset voltage(e.g. 3.3 volts). The reset voltage turns on reset transistor 13 andtransfer transistor 11 which pulls up the voltage on photodiode 10, andfloating diffusion region 12 to an initial reset voltage.

Now the integration phase can commence. During integration, lightenergy, in the form of photons, strikes photodiode 10, thereby creatinga number of electron-hole pairs. Photodiode 10 is designed to limitrecombination between the newly formed electron-hole pairs. As a result,the photo-generated holes are attracted to the ground terminal ofphotodiode 10, while the photo-generated electrons are attracted to thepositive terminal of photodiode 10 where each additional electronreduces the voltage on photodiode 10. Thus, at the end of theintegration step, the potential on photodiode 10 will have been reducedto a final integration voltage where the amount of the reductionrepresents the intensity of the received light energy.

Following the image integration period, the readout period commences.First row select transistor 16 is turned on by applying a selectvoltage, (e.g. 3.3 volts) to the gate of row select transistor 16. Nextthe gate of reset transistor 13, is briefly pulsed with a reset voltage(e.g. 3.3 volts). The reset voltage turns on reset transistor 13 whichpulls up the voltage on floating diffusion 12 to an initial resetvoltage, typically less than or equal to VDD minus the reset transistorthreshold voltage. At this point the depletion region of the floatingdiffusion is at its maximum level and consequently the capacitance ofthe floating diffusion is at a minimum level. The floating diffusionreset voltage on the gate of source of source-follower transistor 15 isthen read out as a reset voltage level. Next the integratedphoto-electrons are transferred from the photodetector to the floatingdiffusion by pulsing the gate of transfer transistor 11. This reducesthe voltage on the floating diffusion 12. The floating diffusion signalvoltage on the gate of source-follower transistor 15 is then read out asa signal voltage level. The signal and reset levels are then subtractedproviding a voltage which represents the total charge collected by cell5.

The maximum number of photo-electrons or maximum signal level typicallyreduces the floating diffusion voltage level by an amount that is allof, or a large percentage of the reset voltage on the floatingdiffusion. As a result, the floating diffusion depletion region widthchanges by a substantial amount compared to the initial depletion regionwidth after reset. This produces a variable floating diffusioncapacitance that is a function of the number of photo-electronstransferred to the floating diffusion. As the number of electronstransferred increases, the floating diffusion depletion region widthdecreases and the floating diffusion capacitance increases. Thisproduces a continuously non-linear transfer function.

The linearity problems created by voltage-dependent capacitance areillustrated in FIG. 3. The vertical axis in FIG. 3 represents thevoltage of the floating diffusion region 12 while the horizontal axisrepresents the light level or integration time. The number ofphoto-electrons that are collected vs. light level or integration timeis a linear relationship. However, since the floating diffusioncapacitance increases as a function of the number of photo-electronscollected, the output signal provided to the column bus 17 from thefloating diffusion region vs. light level or integration time is not alinear relationship.

This relationship can be seen in the solid line A of FIG. 3. Morespecifically, line A represents a continuously non-linear transferfunction. This line has a continuously negative second derivative. LineA has a useable signal range 33 up to the voltage level Vsat′, basedupon a certain percentage deviation from a linear transfer function.This is can be much less than the total signal swing Vsat. While theamount of light energy (e.g., photons) received along the second portion30 of the response line can be calculated, such calculations can resultin higher noise in the rendered image. Therefore, for high image qualityapplications, the APS pixel output is only used for voltages along thefirst portion of 33 and not generally utilized for voltages above Vsat′.

This problem is more severe for the APS pixel shown in FIG. 2. In thiscase the photodiode also functions as the charge to voltage conversionnode, and its diode capacitance comprises a much larger portion of thetotal capacitance associated with electrical node of the gate of thesource follower input transistor. In this case the first portion of thepixel response transfer curve 33 is much smaller than that for the casethe case of the APS pixel shown in FIG. 1.

The invention mitigates these problems by reducing the percentage of thevoltage-dependent capacitance compared with the total capacitanceassociated with the charge to voltage conversion node. Morespecifically, the invention reduces the percentage of thevoltage-dependent capacitance by including a larger voltage-independentcapacitance connected to the charge to voltage conversion node.

For example, in one embodiment, (shown in FIGS. 4a and 4 b), a capacitorC₁ 50 is connected to the charge to voltage conversion node 12. Thecapacitor C₁ 50 is selected to have a very low voltage coefficient toprovide linearity and charge capacity for the reasons stated above. Morespecifically, by adding additional non-voltage-dependent capacitance,the linearity and saturation voltage is increased. In a preferredembodiment the capacitor 50 comprises a polysilicon-polysilicon or otherelectrode-electrode capacitor. Such capacitors exhibit very low voltagecoefficients and provide a capacitance that is independent of thevoltage on the sensing node 12.

The dotted line B in FIG. 3 illustrates the pixel response transferfunction achieved by adding a voltage-independent capacitance inparallel with the floating diffusion. The first portion of the transferfunction (portion 32) that does not deviate from a defined level oflinearity, is increased compared to the prior art. Although the Vsat hasdecreased, since a fixed number of maximum electrons from thephotodetector are converted to a voltage by a larger capacitance, theuseful linear signal level Vsat′, and linear signal transfer function32, can be increased, while the second non-linear portion 31 isdecreased.

Further, with the inventive structure, the overall charge capacity ofthe sensing node is increased, which is useful for cases where a largepixel and large photodetector are required.

Thus, as discussed above, with the inventive structure, the linearsignal response (e.g., portion 32) of the APS is dramatically increasedbecause the overall voltage dependency capacitance of the cell isreduced by adding voltage-independent capacitance device(s).

In addition, as would be known by one ordinarily skilled in the art, acombination of devices can be used to add voltage-independentcapacitance to the APS. For example, multiple capacitors 50 could beused to achieve the necessary level of capacitance.

In another embodiment the invention utilizes a common source amplifier40 as the readout mechanism, rather than the source follower 15 (e.g.,see FIG. 5a and 5 b). The load for the common source amplifier 40 isshown as item 41 along the column bus 17.

The input capacitance of a common source amplifier can be made largerthan that of a source follower amplifier by designing the common sourceamplifier voltage gain to be greater than 1. The input capacitance ofthe common source amplifier 40 is preferably larger than that of thesource follower amplifier so that the sense node junction capacitance isa smaller component of the overall capacitance of the sense node toimprove linearity, and so the total capacitance is larger to providelarger charge capacity on the sense node.

As would be known by one ordinarily skilled in the art given thisdisclosure, the input capacitance of the common source amplifier 40 canbe made (selected) larger by designing the common-source amplifiervoltage gain to provide the desired Miller effect on the gate-draincapacitance and the gate-channel capacitance of the pixel inputtransistor.

Additionally, a combination of the common source amplifier 40 and one ormore capacitors 50 could be used to achieve the reduction in thepercentage of the of voltage-dependent capacitance of the sense node,and the corresponding increase in linear signal response discussedabove.

In addition the capacitor C₁ could be comprise a capacitance to a nodeother than ground, such as VDD.

Thus, the invention produces a greater linear signal response (e.g.,portion 32) to light levels and has a higher voltage saturation levelV_(sat2) because the voltage dependent capacitance of the cell isreduced by adding voltage-independent capacitance devices (40, 50).

While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

PARTS LIST  5 CMOS active pixel sensor cell 10 Photodiode 11 Transfertransistor 12 Floating diffusion region (FD) 13 Reset transistor 14Voltage supply VDD 15 Input transistor (source-follower) 16 Row selecttransistor 17 Column bus 30 Second portion 31 Second portion 32 Transferfunction (first portion) 33 Useable signal range (first portion) 40Common source amplifier 41 Item 50 Capacitors A Solid line B Dotted lineTG Transfer gate RG Reset gate

What is claimed is:
 1. An active pixel image sensor comprised of aplurality of pixels, at least one pixel comprising: a photodetector; atransistor; a charge to voltage conversion region coupled to saidphotodetector and connected to the input of said transistor; and acapacitor connected in parallel with the charge to voltage conversionregion wherein the capacitor is designed to have a low voltagecoefficient.
 2. The device in claim 1, wherein said capacitor provides acapacitance independently of a voltage on said charge to voltageconversion node.
 3. The device in claim 1, wherein said capacitorcomprises a polysilicon to polysilicon double plate capacitor.
 4. Thedevice in claim 1, wherein said capacitor comprises a polysilicon tometal interconnect double plate capacitor.
 5. The device in claim 1,wherein said capacitor comprises a metal interconnect to metalinterconnect double plate capacitor.
 6. An active pixel image sensorcomprised of a plurality of pixels, at least one pixel comprising: aphotodetector; a transistor; said photodetector also operating as acharge to voltage conversion region connected to the input of saidtransistor; and a capacitor connected in parallel with photodetectorwherein the capacitor is designed to have a low voltage coefficient. 7.The device in claim 6, wherein said capacitor provides a capacitanceindependently of a voltage on said charge to voltage conversion node. 8.The device in claim 6, wherein said capacitor comprises a polysilicon topolysilicon double plate capacitor.
 9. The device in claim 6, whereinsaid capacitor comprises a polysilicon to metal interconnect doubleplate capacitor.
 10. The device in claim 6, wherein said capacitorcomprises a metal interconnect to metal interconnect double platecapacitor.
 11. An active pixel image sensor comprised of a plurality ofpixels, at least one pixel comprising: a photodetector; a transistor; acharge to voltage conversion region coupled to said photodetector andconnected to the input of said transistor; wherein said transistor isconfigured to operate as a common source amplifier; and a capacitorconnected in parallel with the charge to voltage conversion regionwherein the capacitor is designed to have a low voltage coefficient.